1. Field of the Invention
The present invention relates to a thermal printing method and a thermal printer w herein print quality may not be degraded by fluctuation in transport speed of a recording sheet through the thermal head that is resulted from variation in friction between the thermal head and the recording sheet.
2. Description of the Related Art
There are thermosensitive recording type thermal printers and thermal transfer type thermal printers. The former heats a thermosensitive recording sheet directly with a thermal head, to cause the sheet to develop color. The latter heats the back of an ink ribbon placed upon a recording sheet to transfer ink to the recording sheet. The thermal printer has a thermal head which has an array of heating elements arranged on a ceramic substrate. The array of heating elements correspond to a line of pixels, and the heating elements are each individually driven to record a dot at a time, so that an image is printed line by line on the recording sheet.
In the thermosensitive recording type and the sublimation ink transfer type thermal printing, one dot constitutes one pixel of the printed image, and has a variable density including a zero level, that is designated by input image data for each pixel. In case of color thermal printing using at least three primary colors, three color dots having variable densities constitute one pixel of a printed full-color image.
As the temperature of the individual heating element varies with the recording density, so do the friction factor between the thermal head and the recording sheet. That is, the friction factor or the coefficient of friction decreases as the heat energy of the heating element increases, and increases as the heat energy decreases. This is because the surface smoothness of the recording sheet increases with an increase of the surface temperature.
The variation in friction factor between the thermal head and the recording sheet results a variation in load applied to the mechanisms such as the transport mechanism of the recording sheet and the support mechanism of the thermal head. Since the mechanisms are slightly deformed by the applied load, the amount of deformation varies with the load. As a result, transport amount of the recording sheet per unit time, i.e., actual transport speed through the thermal head per one drive pulse of the paper transport mechanism, varies with the variation of the friction between the thermal head and the recording sheet.
The fluctuation in actual transport speed results a variation in length of one pixel or dot in the transport direction of the recording sheet. Where the density changes from a high value to a low value, the friction factor increases, and the actual transport speed decreases. With decreasing actual transport speed, the heat energy applied per unit area gets larger, so that the recording density gets higher than expected. Where the density changes from a low value to a high value, the friction factor decreases, and the actual transport speed increases, so that the recording density gets lower than expected.
Accordingly, where the density of the original steeply changes, the tone reproduction is lowered by the transport amount fluctuation. In addition, since the three primary color densities of the original vary differently from each other, the location where the density varies steeply are also different between the colors. Therefore, the frictional variation also results color failures.
In order to keep the print quality from lowering in spite of the transport amount fluctuation, JPA No. 63-296976 discloses a method wherein the actual transport speed is measured for use in correction. However, this method needs an accurate speed measurement device since the fluctuation in transport speed is very small.